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Catalytic decompositionof 2-chlorophenol using an ultrasonic-assisted Fe3O4–TiO2@MWCNT system: influencefactors, pathwayandmechanismstudy

Sina dobaradaran, Ramin Nabizadeh Nodehid, Kamyar Yaghmaeian, JalilJaafari, Maryam Hazrati Niari, Arvind Kumar Bharti, Shilpi Agarwal, VinodKumar Gupta, Ali Azari, Ehsan Ahmadi, Nabi Shariatifar

PII: S0021-9797(17)31171-2DOI: https://doi.org/10.1016/j.jcis.2017.10.015Reference: YJCIS 22885

To appear in: Journal of Colloid and Interface Science

Received Date: 21 August 2017Revised Date: 4 October 2017Accepted Date: 4 October 2017

Please cite this article as: S. dobaradaran, R. Nabizadeh Nodehid, K. Yaghmaeian, J. Jaafari, M. Hazrati Niari, A.Kumar Bharti, S. Agarwal, V. Kumar Gupta, A. Azari, E. Ahmadi, N. Shariatifar, Catalytic decompositionof 2-chlorophenol using an ultrasonic-assisted Fe3O4–TiO2@MWCNT system: influencefactors,pathwayandmechanismstudy, Journal of Colloid and Interface Science (2017), doi: https://doi.org/10.1016/j.jcis.2017.10.015

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Catalytic decomposition of 2-chlorophenol using an ultrasonic-

assisted Fe3O4–TiO2@MWCNT system: influence factors,

pathway and mechanism study

Sina dobaradarana,b,c

, Ramin Nabizadeh Nodehidd, Kamyar Yaghmaeian

d, Jalil Jaafari

e

,Maryam Hazrati Niarif, Arvind Kumar Bharti

g, Shilpi Agarwal

g , Vinod Kumar

Gupta*g

, Ali Azari* h,d

, Ehsan Ahmadid , Nabi Shariatifar

d

a The Persian Gulf Marine Biotechnology Research Center, Bushehr University of Medical Sciences, Bushehr,

Iran b Department of Environmental Health Engineering, Faculty of Health, Bushehr University of Medical Sciences,

Bushehr, Iran c Systems Environmental Health, Oil, Gas and Energy Research Center, Bushehr University of Medical

Sciences, Bushehr, Iran

E-mail: s.dobaradaran@bpums.ac.ir d Department of Environmental Health Engineering, School of Public Health, Tehran University of Medical

Sciences, Tehran, Iran

E-mail: Azari.hjh@gmail.com

E-mail: rnabizadeh@tums.ac.ir

E-mail: k_yaghmaeian@yahoo.com

e School of Health, Guilan University of Medical Sciences, Rasht, Iran

E-mail: jalil.jaafari@yahoo.com

f Department of Environmental Health Engineering, School of Public Health, Jundishapur University of Medical

Sciences, Ahvaz, Iran

E-mail: hazrati.m@ajums.ac.ir

g Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa

E-mail: vinodfcy@gmail.com

h Department of Environmental Health Engineering, School of Public Health, Kashan University of Medical

Sciences, Kashan, Iran. * Correspondence: Ali Azari, Department of Environmental Health Engineering, School of Health, Tehran

University of Medical Sciences, Tehran, Iran. P. O. Box: 6446-14155, Tehran1471613151, I.R. Iran, Phone

No.: +98 21 88779118, Fax: +98 21 88779487E-mail: Azari.hjh@gmail.com

Vinod Kumar Gupta, Applied Chemistry Department University of

Johannesburg, Johannesburg, South Africa

Abstract:

As a reusable sonocatalyst, magnetically separable Fe3O4–TiO2@MWCNT (FMT) was

synthesized by an ultrasound-assisted wet impregnation method and was evaluated in the

removal of 2-chlorophenol (2CP). Physical and chemical properties of the catalyst composite

materials were investigated by All catalysts were systematically characterized using

Transmission Electron Microscopy (TEM), X-ray diffraction (XRD), Scanning Electron

Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), Energy Dispersive X-Ray

Analysis (EDX), Dynamic light scattering (DLS), and N2-physisorption. The efficiency and

kinetics of 2CP removal by FMT-assisted sonocatalysis (FMT-US) was systematically

investigated under various operational parameters i.e. pH, FMT and 2CP concentration,

temperature and ultrasonic power. The results indicated that 0.4 g L-1

FMT dosage, pH 5,

temperature of 35℃ as well as 50 w ultrasound power are the most favorable conditions for

the degradation of the 2CP. Furthermore, both of the superoxide and hydroxyl radicals were

produced in the reaction, however, superoxide radicals were assumed to be the dominating

reactive species for the 2CP degradation, according to the scavenging tests and electron

paramagnetic resonance tests. Moreover, the FMT catalyst exhibited a high reusability and

stability in the US/FMT system during the five repetitive experiments. The intermediate

products were identified by GC–MS, thereby a possible degradation pathway is proposed.

The chemical oxygen demand (COD) and corresponding total organic carbon (TOC) removal

efficiencies were 64.9% and 56.7%, respectively. Finally, toxicity tests showed that the

toxicity of the solution increased during the first 5 min and then decreased significantly with

the progress of the oxidation. The mechanisms of ultrasound irritation enhanced FMT

activation were also proposed.

Keywords: Fe3O4–TiO2@MWCNT nanocomposite, heterogeneous catalysis; 2-clorophenol;

ultrasonic irradiation

1. Introduction

Chlorinated phenolic compounds i.e. 2-chlorophenol (2CP), 4-chlorophenol (4-CP), 2,4-

dichlorophenol (2,4-DCP) and pentachlorophenol (PCP) have been widely known to have

severe detrimental effects on aquatic ecosystem and human health[1]. In the chlorinated

phenolic families, 2CP due to high toxicity, carcinogenic character, persistence in the

environment, and low biodegradability have been listed among priority pollutants by the

United States Environmental Protection Agency[2]. The main routes of entry of 2CP to the

aquatic system are via industrial operations (such as the bleaching of pulp with chlorine,

hydrolysis of chlorinated herbicides and oil refining) or formed as a result of the chlorination

of humic matter during the chlorination of municipal drinking water[3, 4]. They may also be

introduced into the environment during their manufacture and use or through degradation of

other chemicals (e.g., phenoxyakanoic acids)[5]. 2CP exposure is associated with various

health and ecotoxicological risks to the human and aquatic wild life; even very low

concentrations have resulted in harmful effects on the human endocrine system and aquatic

wild life reproduction[6]. Hence, the removal of 2CP from wastewater represents an

emerging environmental concern. A wastewater stream containing 2CP over 200 mg L−1

may

not be treated effectively by direct biological methods[7], therefore decomposition of 2CP

has been examined extensively by photocatalytic [8], electrocatalytic [9], Fenton’s oxidation

[10] and more recently, sonochemical methods [11]. Among the mentioned treatment

technologies, TiO2 photocatalysis because of its good activity, chemical stability, commercial

availability and inexpensiveness has well-known as a promising technology for organic

contamination treatment[12]. However, the separation and recovery of photo catalyst are

difficult, which limit the application of TiO2 slurry reactor in practical application[13] The

recent studies have been focused on the immobilizing TiO2 onto magnetic substrates i.e.

Fe3O4 NPs, which provide a very convenient approach for the separation and recycling of the

photocatalyst[14]. However, Fe3O4 nanoparticles (NPs) are susceptible to air oxidation.

Moreover, directly introducing Fe3O4 NPs as the core of the Fe3O4@TiO2 nanostructure

would produce photo dissolution problems[15]. Many researches indicated that the

photodissolution problem could be prevented by introducing a passivation layer between the

Fe3O4 core and the TiO2 shell[16]. MWCNTs are 1D carbon-based ideal molecules with a

nano cylindrical structure, which can conduct electricity at room temperature with essentially

no resistance (ballistic transport)[17]. While the electrons formed by UV irradiation migrate

to the surface of the MWCNTs, they are easily transported into the conduction band (CB) of

TiO2 which is bound with them[17, 18]. Hence, the increased amount of generated photo-

electrons can decrease the high rate of electron/hole pair recombination, which otherwise

reduces the quantum yield of the TiO2 process[19]. Besides, the use of TiO2 without the

worry about agglomeration at higher concentration, adsorption of intermediate products that

are produce after the reaction, and concentrate the pollutants near TiO2 particles to continue

the degradation process is another important point about MWCNTs[20, 21]. When this

catalyst is used in industrial waste treatment the catalyst cannot absorb the ultraviolet

radiation properly due to the dark nature of the industrial effluent[22]. This problem can

sometimes be overcome by using high power ultraviolet light, which increases both the

energy consumption and equipment´s price [22]. A good alternative to ultraviolet excitation

is ultrasonic irradiation. The process catalyzed by ultrasound is named sonocatalysis. The

ultrasound effect on molecules is related to cavitation, nucleation, growth and implosion of

microbubbles that trap steam/gas[22, 23]. These microbubbles produce areas of high

pressure, which lead to water dissociation and formation of OH radicals. These radicals are

responsible for the degradation process [24, 25]. The ultrasound irradiation induces electrons’

movement on the TiO2 crystal network, increasing the electron-hole pair number and

increasing the OH radical concentration in the reaction medium as well[26]. Moreover, radial

ultrasound may disperse the aggregated catalyst particles, thereby increasing active surface

area. Furthermore, radial ultrasound is also beneficial to the activation of the reused

photocatalysis[27]. Although many studies have addressed this issue that combining

ultrasound with TiO2 can enhance the efficiency of semiconductor mediated degradation of

organic contaminants synergistically, but designed magnetic core-shell sonocatalyst

possesses a uniform size, good structural stability, high surface area, excellent magnetic

separation, and remarkable sonocatalytic performance still remains as a challenge.

According to the above mentioned, the purpose of this report was centered to combination the

advantages of MWCNTs and Fe3O4 to design an effective catalyst, characterization of

prepared catalysts by XRD, EDX, SEM, PDI, DLS, XPS, VSM and TEM techniques,

evaluation the catalytic activity, reusability and stability of catalysts in FMT-US system,

investigation the reaction mechanisms and degradation pathway, determination the amount

of iron and titanium leaching during the degradation process and finally toxicity assessment.

2. Materials and Methods

2.1.. Reagents

Reagents and chemicals used for the study were given in supplementary material S1.

2.2.Synthesis of catalysts

Preparation of Fe3O4 by co-precipitation method, TiO2 by sol-gel route, magnetic Fe3O4–

TiO2 via a sonochemical route, and MWCNT-TiO2 nano-composite using the sol–gel method

was applied to the present work are given separately in supplementary material S1.

2.2.1. Preparation of Fe3O4- MWCNT-TiO2 (FMT) nanocomposites

Magnetic Heterogeneous Fe3O4–TiO2@MWCNT catalyst was prepared using ultrasound-

assisted wet impregnation method is as follows: Different wt% of Fe (III) chloride and

predetermined amount of MWCNT-TiO2 nano-composite were added to 100 mL of deionized

water under continuous stirring for 24 h at room temperature. The resulting mixture then

sonicated for 30 min in the ultrasonic cleaning bath (KQ-300DE, 40 kHz, 300 W). After

mixing, the solution was evaporated using water bath at 100˚C under continuous stirring. The

obtained catalysts were dried for 48 h at 100˚C and then calcined at 500˚C for 4 h.

Subsequently obtained catalysts were named according to the different Fe loading onto TiO2:

Fe3O4 i.e. TiO2 - Fe3O4 (1:1) TiO2 - Fe3O4 (9:1).

2.3.Characterization and analytical method

All catalysts were systematically characterized using Transmission Electron Microscopy

(TEM), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron

Spectroscopy (XPS), Energy Dispersive X-Ray Analysis (EDX), Dynamic light scattering

(DLS), vibrating sample magnetometer (VSM) and N2-physisorption (see supplementary

material S1).

μ

μ

°C; retention time, 0.8 The intermediates

generated during the degradation of 2CP were detected by an Agilent 6890 gas

chromatograph with a 30-m to 0.25-mm HP-5MS capillary column coupled with an Agilent

5973 mass spectrometer (USA). Helium, as carrier gas, was fed into the instrument with a

constant flow rate of 1 mL min-1

. The initial temperature of the oven was set to 40 ℃ for 1

min. Then, it was increased to 300 ℃ at increasing rate of 5 ℃/min and maintained for 3 min.

The Fe and Ti ion concentrations were measured using an inductively coupled plasma mass

spectrophotometer (ICP-MS; Agilent 7500, Ce, Japan) and extent of mineralization during

degradation evaluated by total organic carbon (TOC) content using a TOC- 5000A analyzer

(Shimadzu, Japan). The chemical oxygen demand (COD) was determined (Merck

Spectroquant TR320) by a closed reflux colorimetric method according to Standard Methods

for the Examination of Water and Wastewater (APHA, A., WPCF 1985).

2.4.Reaction procedure: sonocatalytic and photocatalytic activities

The catalytic activities of the FMT composites were determined by the degradation and

mineralization of 2CP in aqueous solution. The catalysts (0.2 g) were suspended in 200 mL

of 2CP solution with a concentration in a glass vessel. The 2CP solutions and NPs

were stirred magnetically in dark for 30 min prior to ultrasonication or UV–visible light to

ensure that the adsorption/desorption equilibrium of the substrate on the catalyst was

achieved as well as effect of adsorption during sonocatalysis or photocatalysis was

eliminated For the degradation process of 2CP, a plastic container (diameter = 20 cm, height

= 10 cm) filled with ice was used to adjustment a temperature environment at around

predetermined amount.

The initial pH of the 2CP solutions was adjusted to 3-11 with HCl (0.1 mol L-1

)

and NaOH (0.1 mol L-1

) using pH meter (Jenway 3510).

HPLC In case of photocatalysis, the prepared suspended solution was

placed in UV–visible chamber (125 W, 198.4 mW S-2

) under stirring, which disperse the NPs

in the solution. After desired time interval of UV–visible irradiation, samples were withdrawn

from the reactor, and changes 2CP concentration were measured. The effects of the dimethyl

sulfoxide (DMSO), carbonate, sulphate and chloride as common inorganic anions on the

degradation performance were investigated by separately adding of their salt in to the

reaction solution.

Meanwhile the effect of 1,4-benzoquinone and tert-butanol as a scavenger on the

sonocatalytic degradation was investigated. The sonocatalytic and photocatalytic activities of

samples in terms of 2CP degradation was reported by using the following equation:

(1)

where, and is the initial and final absorbance of the 2CP solutions after degradation.

The kinetic studies for sonocatalysis and photocatalysis systems were carried out in amber

flasks after optimization of process condition. The catalytic degradation followed the first-

order reaction (eq. 2), where is the apparent-first-order reaction rate constant (min-1

),

is the initial concentration in the bulk solution (mg L-1

) and is the reaction time (min)

The degree of mineralization was evaluated by monitoring the reduction in organic carbon of

the samples using a TOC and COD analyzes

3. Results and discussion

3.1. Characterization of nanocomposites

3.1.1. XRD analysis

The XRD technique (Fig. 1a) was used to determine the crystallographic structure of the

MWCNTs-TiO2(a) and (b) FMT. For the MWCNTs-TiO2 composite, nine distinctive peaks

located at 2θ = 25.4◦, 37.9

◦, 48.1

◦, 54.2

◦, 55.1

◦, 62.8

◦, 69.1

◦, 70.3

◦ and 75.4

◦, which correspond

to the (101), (004), (200), (105), (211), (204), (116), (220) and (215)

, respectively, indicating that the titanium dioxide in the structure of

MWCNTs-TiO2 existed as the anatase (JCPDS: No. 21-1272).

θ

◦ ◦

◦ ◦ ◦ ◦

λ λ

θ β

β ◦

.

3.1.2. TEM analysis

 

a b

c d

e f

MWCNTs-TiO2 and FMT (a), pure TiO2 and Fe3O4 ,

FMT FMT

3.1.3. PDI and DLS analysis

3.1.4. SEM and EDX analysis

a b

c

c

d

e f

3.1.5. XPS analysis

 

a b

c d

XPS spectra for FTM: (a) survey, (b) O1s, (c) C1s, and (d) Fe2p. XPS spectra of FTM after

degradation: (e) survey, (f) Fe2p.

3.1.6. VSM analysis

.

3.1.7. Dispersion analysis

e f

3.2. Degradation procedure

3.2.1.

→ →

→ →

+ US → + (6)

+ → (7)

+ → + (8)

+ → (9)

+

→ + (10)

+ → + (11)

+ US → + (12)

+ → (13)

2CP + free radicals produced→ intermediates → final products (14)

+ UV → (15)

UV+ → + or O + 2H (16)

+ → or + O (17)

→ + (18)

+ → + (19)

→ + (20)

+ + → + (21)

2CP + free radicals produced→ intermediates → final products (22)

+ US or UV→ + + (23)

+ → , + → + (24)

+ → + (25)

2CP + + → degradation product (26)

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

F e 3 O 4

T iO 2

M W C N T

F e 3 O 4 -T iO 2

M W C N T -F e 3 O 4

M W C N T -T iO 2

F M T

U S

U V

U S /F e 3 O 4

U V /F e 3 O 4

U S /T iO 2

U V /T iO 2

U S -F e 3 O 4 -T iO 2

U V -F e 3 O 4 -T iO 2

U S -M W C N T -T iO 2

U V -M W C N T -T iO 2

U S -M W C N T - F e 3 O 4

U V -M W C N T - F e 3 O 4

U S -F M T

U V -F T M

R em o v a l (% )

A d so rp tio n

D e g r a d a t io n

C a ta ly t ic d e g r a d a tio n

comparison the single and composite systems in Sono and photo catalytic removal of 2CP: catalyst

dosage 0.2 g/L, pH 7, Initial concentration 2 mg L-1 under 30 min

º

+ US or UV→ + (27)

+ → +

(28)

+ → (29)

+ →

(30)

→ + (31)

+

+ → degradation product (32)

+ US or UV→ - (33)

- →

- (34)

+ →

+ (35)

+

→ (36)

+ → + (37)

+ +

+ 2CP → degradation product (38)

+ US or UV→ + (39)

+ MWCNT→

+ (40)

+ → + (41)

+ →

(42)

+ → + (43)

+ +

+ 2CP → degradation product (44)

+ US or UV→ + (45)

+ MWCNT→ + (46)

+ → + (47)

+

→ + (48)

+ +

+ 2CP → degradation product (49)

→ + +

– (50)

→ + +

(51)

,

. [73]

3.2.2.

3.2.2.1.

α υ

υ

α υ

(

0 2 4 6 81

01

52

03

0 0 2 4 6 81

01

52

03

0 0 2 4 6 81

01

52

03

0 0 2 4 6 81

01

52

03

0 0 2 4 6 81

01

52

03

0

0 .0

0 .5

1 .0

1 .5

2 .0

2 .5

T im e (m in )

Ct

(mg

L-1

)T iO 2 : F e 3O 4 (1 :1 )

T iO 2 : F e 3O 4 (3 :1 )

T iO 2 : F e 3O 4 (5 :1 )

T iO 2 : F e 3O 4 (7 :1 )

T iO 2 : F e 3O 4 (9 :1 )

Fig 5.

3.2.2.2.

º[38]

0 4 8

15

30 2 6

10

20 0 4 8

15

30 2 6

10

20 0 4 8

15

30 2 6

10

20 0 4 8

15

30

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

T im e (m in )

Re

mo

va

l (%

)

0 .0 5 0 0 .1 0 0 0 .2 0 0 0 .4 0 .6 0 .8 1 g r /L

Fig 6.

0 4 8

15

30 2 6

10

20 0 4 8

15

30 2 6

10

20 0 4 8

15

30

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

T im e (m in )

Re

mo

va

l (%

)

1 2 3 4 5 m g /L

Fig 7.

3.2.2.3.

fi

fi

fi

º º

p H 3 p H 5 p H 7 p H 9 p H 1 1

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

0

2

4

6

8

1 0

1 5

2 0

3 0 m in

Re

mo

va

l (%

)

-0 .5 0 -0 .2 5 0 .0 0 0 .2 5 0 .5 0 0 .7 5 1 .0 0

2

3

4

5

6

7

8

9

1 0

1 1

p H

pH

f-p

Hi

Fig 8 (a)

and (b)

3.2.2.4. Effect of solution temperature and ultrasonic power on degradation

efficiency

℃

℃

a

b

℃

℃

fi

1 5 2 5 3 5 4 5 5 5 2 5 5 0 7 5 1 0 0

5 0

6 0

7 0

8 0

9 0

1 0 0D

eg

ra

da

tio

n (

%)

T e m p e r a tu r e

e ffe c t (C )

U ltr a so n ic

p o w e r (W )

Fig. 9.

pH of 5 and the reaction time

of 15 min

3.2.3. Influence of hydroxyl radical scavengers

ion

, which have hydroxyl radical scavenging

property. In this set of experiments, the initial concentration of 2CP, the catalyst dosage,

ultrasonic power, the concentration of ionic species was constant at 2 mg L-1

, 0.4g L-1

, 50 W

and 5 mg L-1

, respectively. Experimental results illustrated, degradation efficiency of 2CP

decreased in the presence of all used radical scavengers. According to the obtained results,

the degradation efficiency decreased from 100% to 67%, 82%, 73% and 91%, in the presence

of DMSO, chloride, carbonate and sulfate ions, respectively at the reaction time of 10 min

(Fig 10). Statistical analysis (one-way ANOVA) indicated that the diff erence in degradation

ratio between the control group (no scavenger) and the radical scavenger-containing groups

(i.e., DMSO, chloride, carbonate and sulfate ions) were significant (P < 0.01). Among the

scavengers, DMSO was the most eff ective (n = 5). The results confirmed that the free radical

attack mechanism play an important role in the FMT-US-induced degradation of 2CP. The

possible reactions that can occur in the presence of the radical scavengers are as follows:

+ → (52)

+ → (53)

+ → + (54)

+ →

+ (55)

+ →

+ (56)

+ → + (57)

N o sc a ve n g e r D M S O C a r b o n a te S u lp h a te C h lo r id e

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

De

gr

ad

ati

on

(%

)

Fig. 10. 5 mg L-1

:

3.2.4. Mineralization assessment and mean intermediates

fi

fi

chlorinated aromatic rings

less under the same experimental

conditions

2 C P d e g ra d a tio n (% ) T O C re m o v a l (% ) C O D re m o v a l (% )

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

Re

mo

va

l (%

)

Fig. 11.

3.2.5. Rate kinetic modelling

The sonocatalytic degradation mechanism and kinetics of 2CP were studied using Langmuir–

Hinshelwood (L–H) model. Following equation expressed the L–H kinetic model[44, 45]:

(63)

where r is the reaction rate, is the reaction rate constant, is the reactant adsorption

constant, h is the fraction of the surface of catalyst covered by 2CP, and is the

concentration of 4-CP at any time . Eq.63 can be simplified to a first order reaction when

is very low, in which case one has:

(64)

Where =

The integration of Eq. (64) gives:

(65)

where is the initial concentration of 2CP, is the concentration of 2CP at time , is

the first-order reaction rate constant (min-1

), and t is the reaction time (min). By plotting

ln( / ) versus t, the apparent rate constant ( ) can be determined from the slope of the

curve obtained. The time required for concentration of the reactant to drop to half its value is

called the reaction’s half-time or half-life ( ). Half-life for first-order reaction could also be

calculated by:

(57)

In present study, the influence of studied parameters including Fe3O4-TiO2 ratios, Catalyst

dosage, initial 2CP concentration, initial pH, temperature and ultrasonic power on the kinetic

of 2CP degradation was investigated. The values of first-order reaction rate constant k related

to the various parameters along with their regression coefficients R2 are given in Table 1. In

all the experiments, the high regression coefficient (>0.8544) supported the fitting of

sonocatalytic degradation of 2CP by first-order reaction kinetic model. From Table 1, it can

observe that the rate constants strongly depend on Initial pH. For example, at initial pH 5, the

degradation rate of 2CP increased to 0.3717 min-1

, while the amount of k0 in pH 11 was equal

0.0574 min-1

.

Table 1. influence of various parameters on the kinetic of 2CP degradarion.

Parameter Value Equation k0 (min-1

) R2 t1/2(min)

TiO2: Fe3O4 ratios

1: 1 y = 0.0363x + 0.0255 0.0363 R² = 0.971 19.09091

3: 1 y = 0.0464x - 0.0399 0.0464 R² = 0.9949 14.93534

5: 1 y = 0.0627x - 0.0168 0.0627 R² = 0.9894 11.05263

7: 1 y = 0.0526x - 0.0038 0.0526 R² = 0.982 13.1749

9: 1 y = 0.0272x + 0.0143 0.0272 R² = 0.9912 25.47794

Catalyst dosage

0.05 y = 0.0337x + 0.1056 0.0337 R² = 0.8713 20.5638

0.1 y = 0.0769x + 0.0296 0.0769 R² = 0.9098 9.011704

0.2 y = 0.1116x + 0.042 0.1116 R² = 0.9695 6.209677

0.4 y = 0.1917x - 0.2795 0.1917 R² = 0.9617 3.615023

0.6 y = 0.0838x + 0.1603 0.0838 R² = 0.9307 8.26969

0.8 y = 0.0574x + 0.1905 0.0574 R² = 0.9018 12.07317

1 y = 0.0421x + 0.1822 0.0421 R² = 0.8805 16.46081

2CP concentration

1 y = 0.2811x - 0.2124 0.2811 R² = 0.9331 2.465315

2 y = 0.1917x - 0.2795 0.1917 R² = 0.9617 3.615023

3 y = 0.085x + 0.0868 0.085 R² = 0.9717 8.152941

4 y = 0.0656x + 0.0262 0.0656 R² = 0.9871 10.56402

5 y = 0.0272x + 0.0824 0.0272 R² = 0.9032 25.47794

Initial pH

3 y = 0.2228x - 0.2766 0.2228 R² = 0.9704 3.110413

5 y = 0.3717x - 0.3625 0.3717 R² = 0.8544 1.864407

7 y = 0.1917x - 0.2795 0.1917 R² = 0.9617 3.615023

9 y = 0.0574x + 0.1905 0.0574 R² = 0.9018 12.07317

11 y = 0.0249x + 0.1418 0.0249 R² = 0.851 27.83133

Temperature

15 y = 0.052x + 0.0329 0.052 R² = 0.9958 13.32692

25 y = 0.0909x + 0.2067 0.0909 R² = 0.9316 7.623762

35 y = 0.1007x - 0.0124 0.1007 R² = 0.9939 6.881827

45 y = 0.0431x + 0.1434 0.0431 R² = 0.8872 16.07889

55 y = 0.0303x + 0.0629 0.0303 R² = 0.9438 22.87129

Ultrasonic power

25 y = 0.0355x + 0.0896 0.0355 R² = 0.8863 19.52113

50 y = 0.0993x - 0.1258 0.0993 R² = 0.9893 6.978852

75 y = 0.1343x - 0.0835 0.1343 R² = 0.994 5.160089

100 y = 0.1918x - 0.273 0.1918 R² = 0.9649 3.613139

3.2.6. Mechanism of ultrasonic degradation

To characterize the formation of the generated radical species over FMT under simulated

ultrasound irradiation, ESR techniques coupled with DMPO were employed. The typical four

peaks (Fig 12) in the spectra with intensity ratio of 1:2:2:1 could be assigned to DMPO-OHº,

indicating OH radical generated at the initial phase under irradiation. No ESR

signal is observed for these samples in the dark and silent conditions. On the other hands,

electrons in the conduction band can be rapidly trapped by molecular oxygen adsorbed on the

catalyst particle, which is reduced to form superoxide radical anion[46] (E O2/O2-º = -0.16

V). Therefore, the formation of superoxide radicals was also examined by DMPO spin

trapping ESR techniques in methanolic media. As shown in Fig 12, six characteristic peaks of

the DMPO-O2-º adducts were observed on FMT system under ultrasonic irradiation. By

contrast, there are no signals detected in blank test. These results elucidated that superoxide

radicals were efficiently generated under FMT-US system. Based on the results, a possible

sonocatalytic process for the degradation of 2CP under ultrasonic irradiation are proposed:

Upon ultrasonic irradiation, TiO2 undergo charge separation to yield electrons (e-)

and holes (h+). Since the MWCNTs are known as good electron acceptors, the electrons on

conduction band of the TiO2 rapidly transfer to MWCNTs. The negatively charged

MWCNTs (e -) can react with the O2 to produce the active species O2

-º radicals which reacts

with hydrogen ions (H+) to produce hydroxyl radicals (OHº), which oxidizes the adsorbed

2CP directly on the surface. At the same time, the holes from TiO2 react with adsorbed water

to further produce hydroxyl radicals. Finally, the active species (h+, O2

-º and OHº radicals)

oxidize the 2CP molecules adsorbed on these active sites of the FMT system through π-π

stacking interactions and/or electrostatic interaction. Based on the above analysis, the

degradation mechanisms is proposed as follows (schematic S1):

+ US → + (58)

+ → + (59)

+ → + (60)

+

→ (61)

+ → +

(62)

+

+ → degradation product (63)

Fig 12.

During the FMT activation process, Superoxide (O2º-) and hydroxyl radical (•OH) were the

main radicals generated. In order to identify the dominating radicals for 2CP degradation in

the US/FMT system, scavenging experiment was carried out. 1,4-benzoquinone and tert-

butanol (TBA) were used as radical scavengers (quenching agents) to identify the oxidizing

radical species in a FMT-US system[47]. The results of scavenging experiment were showed

in Fig. 13. It can be seen that the addition of Both 1,4-benzoquinone and tert-butanol lead to

decrease the degradation of 2CPin comparison with the no addition of quenching agent

reaction system. These results suggested that radical oxidation (O2º- and OH

º) is the

dominant mechanism in degradation of 2CP by FMT-US system. As depicted in Fig. 13, by

adding TBA, oxidation efficiency reached to 72.5%. However, since 2CP degradation

efficiency decreased to 23.3% in the present of 1,4-benzoquinone, O2º- is considered as the

dominant radical species in the FMT-US system.

N o sc a ve n g e r 1 -4 -b e n zo q u in o n e te r t -b u ta n o l

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

De

gr

ad

ati

on

(%

)

Fig 13. 1,4-benzoquinone and tert-butanol on 2CP degradation using US-FMT system.

3.2.7. Reusability of the sonocatalyst

As well as the excellent sonocatalytic activity, the reusability of sonocatalysts are also

important parameter in practical applications and its economical point of view. To evaluate

the reusability efficiency of the FMT sample, the particle size distribution and magnetic

performance of MST were determined under optimized operational conditions. As a result,

FMT prior to sonocatalytic test had a size distribution in the ranges 20 to 50 nm. After

sonocatalysis process, FMT was separated and dried prior to particle size distribution

analysis. It revealed a size distribution ranging from 39±2 nm (Figure not shown). The

homogeneity in the size distribution might be due to the physical effect of US irradiation[48].

Moreover, magnetic property of FMT after degradation process (Ms= 41.5 emu g-1

) revealed,

sonocatalysts were completely separated by a permanent external magnet. To examine the

reusability of the FMT nanocomposite, the sonocatalytic degradation experiments were

performed with catalyst dosage of 0.4 g L-1

, initial 2CP concentration of 2 mg L-1

and the

reaction time of 15 min at 35℃ temperature. In each run, the sonocatalytic activity of the

recycled FMT nanocomposite exanimated by collecting the nanocomposite from sample

solution. The recovered nanocomposite (thoroughly washing with water and ethanol and

drying) have developed again to degrade the 2CP repeatedly under ultrasonic irradiation. The

results are presented in Fig. 14. As can be seen, the sonodegradation efficiency has negligible

drops from 100 to 94%, 90%, 86%, and 85.96%, respectively during five repeated runs.

These evidence indicates that the FMT nanocomposite are durable and effective for the

remediation of 2CP up to five runs. The partial deactivation of the catalyst is related to some

surface poisoning that may be induced by adsorbed intermediates.

1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

0 .0 0 0

0 .0 2 5

0 .0 5 0

0 .0 7 5

0 .1 0 0

0 .1 2 5

0 .1 5 0

C a ta ly t ic r u n

De

gr

ad

ati

on

(%

) Le

ac

hin

g(p

pm

)

T i

F e

Fig 14. Reusability and leaching of the FMT sonocatalyst during 5 consecutive runs.

3.2.8. Leaching experiment and Toxicity evolution

The leaching of Fe and Ti from the FMT system to the solution was determined during five

repeated runs and the mass of leached was shown in Fig. 14. It was found that, about 0.130

and 0.025 mg L-1

of Fe and TiO2 leached from the catalyst at 5th runs under optimized

condition, respectively. Reutilization tests using FMT revealed some loss in activity (Fig. 14)

from the first to the third run, while it was maintained constant after the third run. Similarly, a

significant decrease in the amount of leached of Fe and TiO2 was observed in the 3rd run and

almost stabilized after that. This suggests that after some initial Fe and Ti lost by leaching,

the catalyst tend to stabilize under continuous use with a significant sonoactivity. However, it

should be noted that the amount of leached iron in US-FMT systems did not exceed the

legislated limit that can be found in waste waters. As previously stated, even though 100% of

2CP was degraded by FMT-US system, however conversion of 2CP was about 55 to 65 %. In

this case, the variation of acute toxicity of product was evaluated by a 48- h immobilization

assay with D. magna. A mortality rate of 60% of D. magna was observed in a raw 2CP

solution. After 5 min reaction, the toxicity reached a maximum value, and 100% of D. magna

was immobilized. The results are probably due to the fact that the by-products of 2CP

exhibited higher acute toxicity [48-60], and thereby acute toxicity increased during the first 5

min. When the reaction time was extended to > 5 min, the by-products decomposed

gradually, leading to the decreased toxicity (55%). Therefore, mare than 5 min reaction time

is necessary to reduce acute toxicity for the 2CP treatment.

4. Conclusions

FMT with average size of lower than 60 nm were synthesized and used as reusable

sonocatalyst for sonocatalytic removal of 2CP. The removal efficiency of US-FMT was

higher than that of other removal systems, and TiO2:Fe3O4 with 5:1 ratio showed the best

sonocatalytic performance with a 2CP degradation efficiency of 86%. The effect of different

operational parameters including the pH, catalyst dosage, initial 2CP concentration,

temperature and ultrasound power was investigated on the sonocatalysis efficiency. The

sonocatalytic activity of FMT was attributed to the generation of more hydroxyl and

superoxide radicals produced. The synthesized sonocatalyst showed good durability because

there was no loss in the removal efficiency of 2CP by sonocatalysis in five repetitive

experiments. The intermediates of 2CP degradation were identified by GC-Mass analysis.

Given the promising results of this study, more research should be carried out on the use of

FMT for the degradation of other organic pollutants in water and wastewater.

Acknowledgment

Financial supports from the Bushehr University of Medical Sciences, Bushehr, Iran [Project

No. 4324] is gratefully acknowledged.

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